Longacre Pro Precision Tire Gauge
Craftsman IR Pyrometer
Tire temperature: it's critical to tire pressure.
If you remember back to your study of physics, you know air expands as it gets warmer. When air expands within an enclosed space, it increases air pressure. Thus, as the tires get warmer for whatever reason, air pressure increases, changing the shape of the tire (although, radials do not change size by a significant amount), increasing its resistance to distortion from driving forces, and altering the size and/or shape of the contact patch on the pavement.
In order to make sure I'm comparing apples to apples, it's important to verify all the tires are the same temperature, which is where my old Craftsman IR Pyrometer comes into play. Although not as fancy or expensive as high-end pyrometers — and very likely less accurate — it should do the job for comparing tire temperature consistency from tire to tire for this review.
In case you were wondering, tire pressure versus temperature is calculated using the Ideal Gas Law, which, because radials don't change significantly in size (thus, volume) and use the same gas, the law can be used with sufficient enough accuracy for gauging tire pressure changes in relation to temperature. Without getting too technical, the Ideal Gas Law is based on four other laws which explain the relationship of the five factors in the Ideal Gas Law. Those factors are temperature, pressure, volume, quantity of gas particles, and a gas constant. As you'll note, I listed those factors in ascending order from least to most obscure. Don't worry about them, they aren't important for this explanation. Here's what is important, if you use the Ideal Gas Law to calculate tire pressure changes to temperature changes, as the NHTSA states on safercar.gov, it's about 1 PSI of change for every 10°F temperature shift. In Longacre's literature, they say to use .8 PSI for every 10°F change. For this review, I'm going with 1 PSI, because I can easily gauge a half-PSI shift for every 5°F — which is the smallest increment marked on the gauge.
All that means is, unless one tire is demonstrably warmer or colder than the others, there shouldn't be an issue — combine that with the fact that tire pressure recommendations are provided for cold, un-run tires, and you get the reason why it's important to test not only when the tires haven't been run, but when they haven't been exposed to sunlight. Especially when they all haven't been exposed to the same amount of sunlight. Photo: Ryan King, 2019.
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